In tests, the immune system's ability to kill different strains of the bugs, including MRSA, was increased up to 1,000 times. The findings were published in the Journal of Clinical Investigation.

The discovery could mark a turning point in the battle against growing numbers of antibiotic-resistant superbugs. One of the most notorious is MRSA, methicillin-resistant Staphylococcus aureus (MRSA), a potentially deadly strain that poses a major threat in hospitals.

Marmite is especially rich in B vitamins, including niacin. But to provide the benefits seen in the study, much higher B3 concentrations would be needed than can be obtained from the yeast extract spread.

The researchers used clinical "megadoses" of niacin far beyond what any normal diet would provide, but similar to those previously given to patients undergoing treatment. However the scientists warned people not to start taking high doses of niacin without medical supervision. In the studies of mice and human blood samples, big doses of the vitamin increased both the numbers and effectiveness of neutrophils - specialised white blood cells that kill and eat harmful bacteria.

Lead researcher Dr Adrian Gombart, from Oregon State University in the United States, said: "This is potentially very significant, although we still need to do human studies. Antibiotics are wonder drugs, but they face increasing problems with resistance by various types of bacteria, especially Staphylococcus aureus.

"This could give us a new way to treat staph infections that can be deadly, and might be used in combination with current biotics. It's a way to tap into the power of the innate immune system and stimulate it to provide a more powerful and natural immune response."

Co-author Dr George Liu, also from Oregon State University, said: "This vitamin is surprisingly effective in fighting off and protecting against one of today's most concerning public health threats."

Dr Gombart stressed that so far there was no evidence to suggest normal diets or conventional-strength B3 supplements could help prevent or treat bacterial infections. Ten years ago he discovered a human genetic mutation that makes people more vulnerable to bacterial infections. Further work showed that niacin has the ability to "turn on" anti-microbial genes that help the immune system fight off bacteria.